On 2015-09-21 16:35, Erkki Seppala wrote:
> Gareth Pye <gareth@cerberos.id.au> writes:
>
>> People tend to be looking at BTRFS for a guarantee that data doesn't
>> die when hardware does. Defaults that defeat that shouldn't be used.
>
> However, data is no more in danger at startup than it is at the moment
> when btrfs notices a drive dropping, yet it permits IO to proceed. Is
> there not a contradiction?
>
> Personally I don't see why system startup should be a special case, in
> particular as it can be very stressful situation to recover from when
> RAID is there just to avoid the immediate reaction when hardware breaks;
> and when in practice you can do the recovery while the system is running
> in systems where short service interruptions matter.
>
The difference is that we have code to detect a device not being present 
at mount, we don't have code (yet) to detect it dropping on a mounted 
filesystem.  Why having proper detection for a device disappearing does 
not appear to be a priority, I have no idea, but that is a separate 
issue from mount behavior.